Get it right from the start

Many factors can influence a material recovery facility’s (MRF’s) throughput—the composition and moisture content of the inbound material stream, the speed and efficiency of optical sorters or the number of manual sorters involved. MRF operators might upgrade their optics or install robotics to try to boost efficiencies.

But improving throughput also involves evaluating tipping floor management and how well materials are fed to the system. While sometimes overlooked, these factors are vital to ensuring smooth operations.

During Recycling Today’s 2021 MRF Operations Forum, which was hosted online Oct. 18-19, Senior Recycling Manager Mark Henke of Phoenix-based Republic Services and Executive Vice President of Sales Engineering David Marcouiller at Machinex, Plessisville, Quebec, offered some key takeaways for MRF operators to consider regarding tipping floor operations and feeding their systems.

Designing the tipping floor

When designing or making changes to a tipping floor, Marcouiller advised MRF operators to seek outside input from equipment suppliers “as early as possible in the process.”

He said, “It’s usually with great pleasure that we’ll assist you, give you some references, conceptualize and share good insights with you.”

Marcouiller added that MRF operators should be ready to provide their suppliers and any other design consultants with basic details: What is traffic flow like around the building? What is the site plan? If the building is in place, where will the tipping floor be? If the building is not yet in place and it’s a new MRF, what shape does the operator want the tipping floor to be? What kinds of materials will be processed? Will the MRF operate one shift or two? Finally, what are the MRF’s plans for growth?

“All of these things are very important to designing,” Marcouiller said. “Early collaboration is key. We like to have brainstorming sessions with your building contractors and your operator. If you’re a municipality and you already have an operator, we’d like to get involved with them, understanding what their preferences are and how they operate. If you’re a private [company], it’s always good to sit with your current staff and find out the lessons learned from the past and share those ideas.”

Once the basic questions are answered, best practices should be kept in mind when designing a tipping floor.

The shape of a tipping floor can vary: It can be linear, square or L-shaped. Marcouiller said these different shapes all have pros and cons, but ultimately the MRF’s site plan or property will determine the layout that makes the most sense for the space.

In addition to tipping floor layout and shape, Henke said MRF operators will want to make sure the tipping floor has more space than it needs in case of downtime or future growth. He suggested ensuring the tipping floor can handle at least two full days of material volume.

“You need that capacity on your tip floor so that you can have enough space so you don’t go outside the confines of the tip floor, which obviously can lead to a compliance issue,” Henke said. “We often think that maybe this is overlooked or doesn’t get the attention it needs during your design phase.”

When building a new MRF, it might cost more to build a larger tipping floor, but Marcouiller said the expense can pay off in the long run. “A lot of times this will serve you well in the future when you have extra capacity for minimal upfront capital investment.”

Additionally, Henke said MRF operators should plan to include multiple access doors to the tipping floor in case multiple loads arrive at the same time.

“You always seem to get material in during about an hour to an hour-and-a-half period of time,” he said, noting that haulers will dispatch their trucks at a certain time.

“It would be great if they could alternate that, so we have more of a distribution of the trucks as they come in, but we do have to manage that rush,” Henke said. “And that’s why we have multiple access doors.”

He added that MRFs that use walking floors in the tipping area should dedicate one access door solely to the walking floor to help manage the flow.

One final consideration would be whether to incorporate an on-site trailer tipper. Although they eat up space and add to the overall design cost, Henke said they offer a MRF more flexibility for unloading.

“A tipper can come in and within five to 10 minutes tip loads in a dedicated location, and they’re out of the way of other inbound trucks,” Henke said. “They increase the initial expenses upfront. But if you talk to people that operate them, they figure that within a couple of years they do pay for themselves because of reduced times on-site.”

© SOLOTU / stock.adobe.com

Feeding the system

Once the tipping floor is designed, a MRF operator needs to determine how to load and feed the system.

Operators should know the system speed and choose a loader that will adequately load the volume of material needed to meet that speed. MRF operators can use a front-end loader equipped with a bucket or a grapple. There are pros and cons to both of these approaches, Henke said.

“A grapple gives you a little more or better payload,” Henke said. “There’s less spillage. It also allows you to work the material quite well on the tip floor.”

The downside to a grapple is that it is stationary requires another piece of machinery to feed it, he said. “If you were going to have two units on the tip floor anyway, this might be a good option. It also allows you to pick and choose some of the heavier items that you see within the stream that you don’t want to introduce to your drum feed unit or your process downstream.”

A MRF’s drum feeder should be designed to consistently meter the flow of the entire system. Feeders also tend to fluff material that comes in. As the loader works with material on the tip floor, it should provide the drum feeder with a good, homogeneous mix of materials. This will ensure a better introduction of materials downstream in the system, Henke said.

“If you underfeed the system, you’re not utilizing your end components to their maximum capacity. If you overfeed the system, it’s very important to understand that you’re also overfeeding your opticals or your eddy currents at the end,” he explained, adding that this is one reason why opticals or eddy currents might not be performing well at an existing MRF.

“You want a good, consistent feed,” he added. “Work with your tip floor operator and make sure he or she is consistently feeding it to a certain level.”

A successful system

Tipping floor management is key when feeding the entire system. A big part of this management means knowing the inbound material stream, which will dictate what methods work best.

Henke said operators need to know the basic characteristics of materials that end up on the tipping floor: Is the material wet? How much residue comes in? What is the blend of commercial to residential materials?

Henke noted that a MRF’s material stream and characteristics likely will change over time. For example, in the past, old corrugated containers (OCC) made up only 10 to 15 percent of the inbound stream. Today, he said, a MRF might receive 20 to 25 percent OCC.

Henke said material streams keep evolving as packaging changes. “A cubic yard of residential material today probably weighs a little less than it did just a few years ago.”

Knowing that inbound stream will help to determine the MRF’s labor needs for the tipping floor and later for quality control. Also knowing the weight of the inbound material stream will help to determine what speed makes the most sense to feed the system. Henke advised operators to weigh some cubic yards of material on the tipping floor to determine that average.

“For example, if you have a 30-cubic-yard-capacity system, and you’re rated at 30 tons per hour and material weighs 300 pounds per yard, you are achieving 30 tons per hour if the drum feed empties at about six minutes, which is a typical flow rate,” he explained. “However, if everything else stays the same, and you run that same 30 yards and it’s only 250 pounds per yard, you’re actually going at less than 25 tons an hour. That all comes into play in your throughput capacity.

“So, understand the characteristics of the material,” he concluded. “It changes over time.”

The author is co-managing editor of Recycling Today and can be reached at msmalley@gie.net.